86 PRINCIPLES OF EMBRYOLOGY 



of an intact egg (Fig. 5.4). They continue developing in a vegetative 

 v/ay, and, further, swing some of the neighbouring animal material into 

 line with them, so that it too develops as gut. The animal gradient of the 

 egg is exhibited by the fact that the nearer the animal pole the graft is 

 placed, the smaller is the gut formed. 



It is important to note the similarities, and also the differences, between 

 the production of a gut by the action of the grafted micromeres on the 

 animal cells in the above experiments, and the 'embryonic induction' 

 which is most strikingly seen in vertebrates. In both cases, a part of the 

 embryo, when grafted into an abnormal situation, causes the material 



Figure 5.3 



A meridional half of the i6-cell stage (uncoloured, on left, containing the 

 whole an-veg axis) is combined with the animal half of the same stage 

 (shaded). A normal embryo develops, shown in section on the right. (After 



Horstadius.) 



surrounding it to develop in a way which it would not normally have 

 done. In true embryonic induction, however, the graft induces from its 

 surroundings something of a different nature to itself; for instance, in the 

 vertebrates, mesoderm can induce the formation of neural tissue, or in 

 Linmea the gut can induce the formation of the shell-gland. In the echino- 

 derms, the micromeres induce something which is either normally derived 

 from micromeres or from the next most vegetative material; we are 

 dealing with a tendency for the completion of the graft at the expense of 

 its surroundings; moreover, these surroundings clearly play an important 

 part in determining the nature of the structures induced. In the verte- 

 brates, as we shall see, induction may sometimes show certain features 

 which indicate a similar assimilative or individualising tendency, but it 

 can also be a much more independent process, the inducer provoking the 

 appearance of something which it does not in any way need in order to 



